1. Field of the Invention
The present invention relates to a mesoporous carbon material, a fabrication method thereof and a supercapacitor using the mesoporous carbon material as an activated material, and more particularly to a method for fabricating a mesoporous carbon material containing carbon nanotubes (CNTs) and/or metal particles and/or metal oxide particles.
2. Description of the Related Art
Recently, development of supercapacitors for the load leveling of electric power sources, including batteries (e.g. rechargeable cells) and fuel cells has become very important, for new mobile communication and electric vehicles that require high pulse power. By connecting in parallel electrochemical capacitors having excellent power output to batteries or fuel cells having high energy density, it is possible to satisfy pulse power output needs and extend the lifetime of batteries and fuel cells.
In general, electrochemical capacitors are classified into electric double-layer capacitors (EDLCs) and pseudo capacitors. EDLCs store electricity by charging ions on electrolytes and electrons on electrodes, respectively, at an electric double layer formed at an electrode/electrolyte interface. The pseudo capacitors store electricity near the electrode surface by using faradaic reaction.
The electric double layer capacitor is composed of an equivalent circuit wherein double-layer capacitance and equivalent series resistance (ESR) are connected in series. The double-layer capacitance is proportional to the surface area of the electrode, and the ESR is the sum of electrode resistance, electrolyte solution resistance, and electrolyte resistance in the electrode pores. The electric charges stored in the double-layer capacitance decrease as the charge/discharge rate increases, wherein the ESR determines the degree of storage decrease. Namely, the storage amount of the charges decrease as the ESR increases and such phenomenon becomes larger as the charge/discharge rate increases.
Generally, the electrode materials for electric double layer capacitors should have the following characteristics: (1) high surface area for high double-layer capacitance; (2) high electrical conductivity for low electrode resistance and (3) low resistance from the electrolyte in the pores of electrode.
Currently, powder and activated carbon fiber forms have been used as the electrode material of EDLCs, but these activated carbons have the following shortcomings, when related to the above-mentioned requirements for EDLCs.
First, the activated carbons are composed of irregular and tortuous micropores (below 2 nm), mesopores (2 nm to 50 nm) and macropores (over 50 nm), which hinders performance of conductivity of the electrode material of electric double layer capacitors. Thus, it is difficult to improve the efficiency of conductivity to be greater than 50%. Also, electrolyte wetting of the micropores is not so easy, and the surfaces exposed in the micropores may not be utilized for charge-discharge capacity. Moreover, even if electrolyte wetting of the micropores is sufficient for ionic transferring in the small and tortuous pores, it does not substantially increase surface area, thus, negating one of the advantages belonging to electric double layer capacitors. Also, if the pores are irregular and tortuous, the charge-discharge capacity and ionic transferring rate capability are further hindered. It is generally accepted that pore sizes greater than 1 nm is desirable for the electrode materials of electric double layer capacitors in an aqueous electrolyte media, and pore sizes greater than 2 nm for those in an organic electrolyte media.
Second, the activated carbons have low electrical conductivity. Carbon materials with high conductivity can be achieved by a thermal treatment process for graphitization. However, the process drastically decreases the specific surface area, which provides the electric double layer surface. Conductive additives such as carbon black or metal powder can be mixed to activate carbons to increase electric conductivity, which in turn, decreases ESR, which, however, will also decrease capacitance per weight or volume. Moreover, the conventional method does not improve the conductivity for the carbon materials.